Switched sim to another phone, now no service

Like the text said, visit an ATT store or call. Your account needs to be updated with the imei of the iPhone. That is not something you can do yourself. It either happens automatically or you need an ATT person to do it.

But I put the sim back in the original working phone so it should work again. I entered the imei of the original phone, and it says imei  already registered. 

Ok, perhaps it’s not the imei updating, it may be something else. Whatever it is, it’s not something you can fix yourself. 

From what I've read once you have your account suspended you have to call ATT.  Just putting it into a valid phone doesn't fix it.

Yes, what @kevink  wrote. 

So I called 611, as this is the phone directed me to when I went to make a call, since it can't make any calls.  After 33 minutes on hold, the person asks all my info but can't find the account.  Then ask if it's prepaid, I say Yes.  She says, Oh, there's a different number you need to call, 800-901-9878.  Of course, the phone won't allow me to call that number, so I had to go borrow someone else's phone to call.  However, I got a live person within 5 minutes and he confirmed the line was suspended due to the incompatible phone and he had it re-instated in 3 minutes.  So he gets a perfect 10, ATT gets a 0 for having websites with old info, non-existing links, and not setting their systems up to distinguish between prepaid and postpaid. I spent a couple hours searching their websites, both for a solution and some indication of what was going on.  If they want to automate everything and make it self service, they need it to be comprehensive and user friendly, not run you around in circles!

611 works for prepaid too. Why you got a postpaid agent, I don’t know but you could have simply asked them to transfer you. 

They said pre-paid account services is at a different phone number you have to call, here it is, no offer to transfer.  It’s ok, gives me a good idea  of what to expect from CS. Maybe my other 4 lines will stay at T-Mobile rather than go back to ATT.